1. Field of the Invention
The present invention relates to an image processing technique for reducing an aberration component in an image produced by image capturing.
2. Description of the Related Art
Images produced by image pickup apparatuses such as digital cameras include a blur component (image blur component) that is a component of image degradation caused by various aberrations of an image capturing optical system (hereinafter simply referred to as an “optical system”) such as spherical aberration, comatic aberration, field curvature and astigmatism. Such a blur component is generated because a light flux emitted from one point of an object forms an image with some divergence on an image pickup surface, the light flux being normally converged at one point if there is no aberration or diffraction.
The blur component herein is optically expressed as a point spread function (PSF), and is different from blur caused by defocusing. Moreover, color blur in a color image caused by longitudinal chromatic aberration, chromatic spherical aberration or chromatic comatic aberration of the optical system can be said to be a difference of blurring degrees for respective light wavelengths. Furthermore, color shift in a lateral direction caused by chromatic aberration of magnification of the optical system can be said as position shift or phase shift due to differences of image capturing magnifications for respective light wavelengths.
An optical transfer function (OTF) obtained by Fourier transform of the PSF is frequency component information of aberration, and is shown by a complex number. An absolute value of the OTF, that is, an amplitude component is called an MTF (Modulation Transfer Function), and a phase component is called a PTF (Phase Transfer Function). The MTF and the PTF respectively show a frequency characteristic of the amplitude component and a frequency characteristic of the phase component of the image degradation by the aberration. The phase component is defined as a phase angle by the following expression:PTF=tan−1(Im(OTF)/Re(OTF))where Re(OTF) and Im(OTF) respectively show a real part and an imaginary part of the OTF.
As described above, since the OTF of the optical system degrades the amplitude component and the phase component of the image, each points of the object in the degraded image is asymmetrically blurred like comatic aberration.
The chromatic aberration of magnification is generated by acquiring differences of image-forming positions of the respective light wavelengths due to differences of image-forming magnifications for the respective light wavelengths as color components such as R, G and B according to a spectrum characteristic of the image pickup apparatus. Accordingly, not only the differences of the image-forming positions among the color components (R, G and B), but also differences of image-forming positions of respective wavelengths in each color component, that is, image spread due to the phase shift are generated. To be exact, the chromatic aberration of magnification does not generate simple parallel color shift. However, description below will be made on the assumption that the chromatic aberration of magnification generates the color shift.
As a method for correcting (reducing) the degradation of the amplitude (MTF) and the degradation of the phase (PTF), there is known a correction method that uses information on the OTF of the optical system. This method is referred to as “image restoration”, and a process for correcting (reducing) the degradation of the image by using the information on the OTF of the optical system is hereinafter referred to as “an image restoration process” or simply as “image restoration”. There is known one image restoration process that performs convolution of an image restoration filter on an input image, the image restoration filter having a characteristic inverse to that of the OTF.
Moreover, there is known a method for correcting only the color shift in the image caused by the chromatic aberration of magnification, which performs a geometric (coordinate) transform process and a pixel interpolation process on pixel signals. For example, Japanese Patent Laid-Open No. 06-113309 discloses a method of correcting the color shift caused by the chromatic aberration of magnification for each of R, G and B according to a focal length of the optical system.
However, the method disclosed in Japanese Patent Laid-Open No. 06-113309 allows the color shift to remain due to insufficient correction or excessive correction of the color shift when the chromatic aberration of magnification is changed due to manufacturing variation of the optical system or variation of a spectrum characteristic of a light source in image capturing.
In regard to this point, a correction method disclosed in Japanese Patent Laid-Open No. 2006-020275 calculates a correlation of two color components included in RAW data to detect a color shift amount, and calculates chromatic aberration of magnification of an optical system on the basis of the color shift amount. That is, the method detects an actual color shift amount and performs color shift correction using the detected actual color shift amount, thereby making it possible to correct the color shift well according to the change of the chromatic aberration of magnification.
However, there is a case where the color shift correction cannot be sufficiently performed even by using the correction method disclosed by Japanese Patent Laid-Open No. 2006-020275. As described above, the chromatic aberration of magnification is generated due to not only the differences of the image-forming positions among the color components but also the image spread caused by the differences of the image-forming positions of the wavelengths in each color component, that is, the phase shift. For example, a G channel image that is an image of one of R, G and B color components includes a phase shift corresponding to chromatic aberration of magnification in a wavelength range in which a spectrum transmittance characteristic of a color filter of G has sensitivity. Thus, the image spread due to influence of the chromatic aberration of magnification of each wavelength that remains in each color component cannot be corrected even if the color shift among the color components can be corrected by the correction method disclosed in Japanese Patent Laid-Open No. 2006-020275.
In other words, the color shift correction performed by means of correction of image magnification of an image is locally equivalent to parallel translation of the image, so that it only corrects a linear component of the PTF which changes linearly with respect to frequency. Since an image of each color component includes asymmetric aberration such as comatic aberration, the PTF has a non-linear frequency characteristic. Thus, the parallel translation of the image of each color component cannot correct the asymmetry. Accordingly, in order to highly accurately correct the color shift due to the influence of the chromatic aberration of magnification of each wavelength that remains in each color component, it is necessary to also correct a non-linear component of the PTF which changes non-linearly with respect to the frequency.